This invention relates to security apparatus, and in particular to theft detection apparatus for detecting the presence of a saturable magnetic tag which is usually hidden in an article which is intended to be protected.
In recent years a class of theft detection apparatus has become popular in which a tag or strip hidden in an article to be protected is detected. While there are variations of the kind of tag to be detected (some being comprised of resonant circuits etc.), there have been a number of designs and improvements on the designs based on the detection of specific harmonics or groups of harmonics of signals which are stated as being generated by the tag. This class of detector is derived from an invention by Pierre Arthur Picard which is described in French Patent No. 763,681, issued Nov. 10, 1933.
In the invention described in that patent, thin strips of material having a high magnetic permeability are repeatedly driven in and out of saturation in the presence of an alternating magnetic field. A receiving antenna receives magnetic fields which are said to be generated by the strip of material, which alternate at frequencies which are harmonics of the original transmission frequency. A receiver connected to the receiving antenna is tuned to detect signals at one or more of the harmonic frequencies produced by the target strip, and an alarm is activated when such detection takes place. The specific frequencies of the harmonics designate the presence of the specific tag material.
Improvements to that system are described U S. Pat. No. 4,123,749 relating to the continuous rotation of the transmitted magnetic field, U.S. Pat. No. 3,983,552 which relates to demagnetization of a control element associated with the strip, and which detects predetermined harmonics, U.S. Pat. No. 3,665,449 which detects signals of predetermined frequencies and having an applied field of predetermined magnitude, U.S. Pat. No. 3,631,442 which relates to generating at least two oscillating electromagnetic fields at different frequencies, U.S. Pat. No. 3,790,745 which performs analysis of the harmonic components of the received frequencies, U.S. Pat. No. 3,754,226 which relates to a form of transmitting antenna, U.S. Pat. No. 3,820,103 which detects a predetermined harmonic signal generated by the tag or marker, U.S. Pat. No. 4,215,342 which utilizes a sub-resonant tag, U.S. Pat. No. 4,158,434 which relates to the direction of the lines of magnetic saturation of the tags or labels, and U.S. Pat. No. 4,298,862 which relates to the use of amorphous ferromagnetic materials which generate magnetic fields having "tones" which are retained after the marker or tag is flexed or bent.
In the aforenoted patented inventions, the markers or tags to be affixed to the goods are specifically utilized to generate signals which are harmonics of the frequency of the magnetic field which is applied to a detection region or zone. Many of the inventions are directed to refinements for maximizing the possibility of detecting certain ones of the harmonics which are said to specifically identify the unique presence of the strip or tag, or to thereby reject erroneous identification of other metallic objects such as belt buckles, jewelry, other metallic articles of various constitution carried by a person, etc.
Nevertheless it has been found that such apparatus still can generate frequent false alarms, often caused by materials of similar magnetic constitution as the real tag to be detected. As was indicated by Picard, the size or shape of the tag merely varies its amplitude; the constitution of the materials identifies the harmonic content of the signal which is generated. Thus the detection of similar material objects as the strip, marker or security tag has been found to be virtually unavoidable.